The effect of light intensity during growth of Sinapis alba on the electron-transport components

Abstract

Summary Sinapis alba (white mustard) plants were grown under high light (2 × 105 erg/cm2 · sec) and low light (2 × 103 erg/cm2 · sec) conditions, and the effect of light intensity during growth on the electron-transport components and the photochemical activities of the chloroplasts were studied. The P700 content in plants grown at both high and low light is quite high; the Chl/P700 ratio is about 210 in the chloroplasts from both types of plants. The P700 content in these plants shows no correlation with their maximum photosynthetic efficiency. In high-light plants, the concentrations of P700, ferredoxin and cytochrome f are comparable, namely, one of each of these components is present in each electron-transport pathway. In low-light plants, the P700 concentration is about three times higher than that of cytochrome f or ferredoxin. It is therefore suggested that most of the P700 in low-light plants is involved in a cyclic electron flow which is coupled to phosphorylation. The D-1 and D-10 fragments from the high-light-plant chloroplasts show a substantially higher photosystem-II activity than the corresponding fragments from the low-light plants. This is consistent with our previous finding that the plastoquinone concentration in high-light plants is twice as high as in low-light plants. The higher plastoquinone concentration and the higher photosystem-II activity in the high-light plants may be related to a higher concentration of photosystem-II reaction-center chlorophyll or a larger plastoquinone pool in these plants.